Far-right leader Marine Le Pen, who stunned France by seizing almost a fifth of presidential first round votes, said she was waiting for answers from President Nicolas Sarkozy before telling her supporters how to vote in a run-off.

After Le Pen took third place in Sunday's ballot with the National Front's top score in a national election, centre-right Sarkozy and Socialist front-runner Francois Hollande have courted her voters, who may decide the May 6 second round result.

Sarkozy's overtures have been more direct, saying that he respects National Front voters and would not criticise a vote for a party which has long been stigmatised. Hollande has said he understood voters who wanted to express their frustration at a stagnant economy and unemployment running at a 12-year high.

The president on Wednesday ruled out any deal with Le Pen which would give the far-right positions in the cabinet or help them win parliamentary seats in June's legislative elections.

But Sarkozy has yet to say whether he would advise supporters of his UMP party to vote Socialist rather than for the National Front in the second round of the June legislative elections to keep the far-right out of parliament.

"In case of a run-off between the National Front and a Socialist, will the UMP party and the president prefer to have one of my deputies or a Socialist deputy elected?" Le Pen said on RTL radio.

"I still don't have an answer to those questions, I am waiting. That's a question my voters want to know about," she said. "How I express myself will depend on the response."

Le Pen has said she would give her view on the presidential second round choice at the National Front's traditional "Joan of Arc" May Day rally, but senior aides have suggested she was highly unlikely to endorse either candidate explicitly.

Le Pen, who took over the party founded by her ex-paratrooper father Jean-Marie in January last year, has said she hopes to profit from an implosion of the mainstream right.

The prospect of Hollande winning power has sent jitters through financial markets as the 57-year-old has pledged to renegotiate a German-inspired budget discipline pact for Europe, putting him on a collision course with Berlin.

An opinion poll showed two-thirds of Sarkozy supporters want him to break with past policy and strike an alliance with the National Front after Le Pen's 17.9 per cent score on Sunday made her 6.4 million backers key to the presidential run-off.

Most polls show Hollande comfortably winning on May 6 by around 10 percentage points. He is expected to win the vast majority of far-left votes and much of the centrist support.

Sarkozy needs about 80 per cent of Le Pen voters behind him to avoid defeat, according to analyst estimates and a Reuters calculator. But surveys conducted during or after Sunday's first-round presidential vote found that between only 44 per cent and 60 per cent of Le Pen voters plan to switch to Sarkozy in round two, down from about 70 per cent in 2007.

In a setback to Sarkozy, centrist Francois Bayrou, who came fifth with 9.1 per cent of the vote, accused the president of being "absurd and offensive" in comparing his voters with those of Le Pen. In an open letter to both candidates on Wednesday, he called for more civil, clean and moderate politics, appearing to lean towards Hollande without explicitly endorsing him.

Hollande said Bayrou was implicitly criticising the president's courting of the far right. "Nicolas Sarkozy has broken the rules," he told France Info radio. "He has understood that Sarkozy has raced to catch votes since the first round."

A big vote for Sarkozy by National Front supporters would make it mathematically possible for him to win a fresh five-year term.

Its strong showing on Sunday has also given the National Front faith it can win seats in parliament for the first time since 1986, when a brief experiment with proportional representation gave it 35 deputies.

Based on Sunday's results, the party could reach the second round in up to 345 of the 577 constituencies in the parliamentary election, splitting the right-wing vote.

Several of Sarkozy's top cabinet members and advisers have ruled out any alliance with the National Front, although they do not rule out the possibility of the UMP letting the party fight solely against the Socialist party in certain constituencies.